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72 2 Image formation

(a) (b)

Figure 2.27 Primary and secondary colors: (a) additive colors red, green, and blue can be mixed to produce
cyan, magenta, yellow, and white; (b) subtractive colors cyan, magenta, and yellow can be mixed to produce red,
green, blue, and black.

Later on, you may have learned about the additive primary colors (red, green, and blue)
and how they can be added (with a slide projector or on a computer monitor) to produce cyan,
magenta, yellow, white, and all the other colors we typically see on our TV sets and monitors
(Figure 2.27a).
Through what process is it possible for two different colors, such as red and green, to
interact to produce a third color like yellow? Are the wavelengths somehow mixed up to
produce a new wavelength?
You probably know that the correct answer has nothing to do with physically mixing
wavelengths. Instead, the existence of three primaries is a result of the tri-stimulus (or tri-
chromatic) nature of the human visual system, since we have three different kinds of cone,
each of which responds selectively to a different portion of the color spectrum (Glassner 1995;
Wyszecki and Stiles 2000; Fairchild 2005; Reinhard, Ward, Pattanaik et al. 2005; Livingstone
2008). 18 Note that for machine vision applications, such as remote sensing and terrain clas-
sification, it is preferable to use many more wavelengths. Similarly, surveillance applications
can often benefit from sensing in the near-infrared (NIR) range.

CIE RGB and XYZ
To test and quantify the tri-chromatic theory of perception, we can attempt to reproduce all
monochromatic (single wavelength) colors as a mixture of three suitably chosen primaries.
(Pure wavelength light can be obtained using either a prism or specially manufactured color
filters.) In the 1930s, the Commission Internationale d’Eclairage (CIE) standardized the RGB
representation by performing such color matching experiments using the primary colors of
red (700.0nm wavelength), green (546.1nm), and blue (435.8nm).
Figure 2.28 shows the results of performing these experiments with a standard observer,
i.e., averaging perceptual results over a large number of subjects. You will notice that for
certain pure spectra in the blue–green range, a negative amount of red light has to be added,
i.e., a certain amount of red has to be added to the color being matched in order to get a color
match. These results also provided a simple explanation for the existence of metamers, which
are colors with different spectra that are perceptually indistinguishable. Note that two fabrics

18 See also Mark Fairchild’s Web page, http://www.cis.rit.edu/fairchild/WhyIsColor/books links.html.